1. Field of the Invention
The present invention relates to a printer head drive system for a printing apparatus such as an ink-jet printer, and particularly to a printer head drive system for a printing apparatus in which a head unit and a power supply unit for supplying electric power to the head unit are connected by power supply conductors.
2. Description of Related Art
In a printer head drive system for a printing apparatus in which a head unit and a power supply unit for supplying electric power to the head unit are separately located away from each other, both of the units are connected by way of electric power supply conductors formed on a flexible printed circuit board (FPC).
In one drive system shown in FIG. 13, an ink jet type printer head 1 and a head driver IC (integrated circuit) 2 for electrically driving the printer head 1 are provided in a head unit 3. This head unit 3 is mounted on a carriage held movably for printing operation in the printing apparatus. A power supply unit 4 for supplying electric power to the driver IC 2 is provided on the fixed body of the printing apparatus. The driver IC 2 and the power supply unit 4 are connected by power supply conductors 5. The power supply unit 4 has a constant voltage source which comprises a power transistor 6 as a control element, voltage dividing resistors 7 for detecting an output voltage of the power supply unit 4, a reference voltage source 8 and an error detector 9 for controlling the power transistor 6. A control signal is supplied to the driver IC 2.
In this system, the printer head 1 may be a bubble jet type shown in FIG. 14 or may be a piezoelectric element (PZT) type. In case of the bubble jet type, each heater 11 is considered as a resistance member and is energized to generate a steam pressure for ink jetting operation.
In the above arrangement, electric power is supplied between output terminals a and b and an output voltage corresponding to a desired voltage of the reference voltage source 8 is generated between terminals c and d by the power transistor 6. Here, the voltage dividing resistors 7, reference voltage source 8 and error detector 9 provides a negative feedback control. The electric power thus controlled is supplied between terminals e and f serving as power input terminals of the driver IC 2 through the power supply conductors 5. The driver IC 2 selectively supplies the electric power supplied from the power supply unit 4 to individual driving elements within the printer head 1 in response to the inputted control signal.
In another control system shown in FIG. 15, the power supply unit 4 comprises a power amplifier 41 and a pulse generator 42 but has no feedback control. The head unit 3 may be the bubble jet type or the PZT type shown in FIG. 16. In case of the PZT type in which the pressure within ink channels are changed for ink jetting operation, each PZT is considered to be a capacitance member 12.
In either system, the power supply conductors 5 for supplying the electric power from the output terminals c and d of the power supply unit (fixed side) 4 to the head unit (movable side) 3 includes a resistance (R) and an inductance (L) as shown by dotted lines. Although R and L are shown between terminals c and e and terminals d and f in FIG. 13, in a lumped constant manner, R and L exist actually along the length of each conductor in a distributed constant manner.
When ink is to be jetted simultaneously from a number of nozzles, electric current flows to a plurality of channels simultaneously, and power source current is expressed as follows.
I=V.multidot.N/Rh: in the case of using heaters 11, wherein I, V, N and Rh represent power source current, power source voltage, number of simultaneously-driven ink jet nozzles and nozzle resistance value (heater resistance), respectively.
I=V.multidot.N/Ron: in the case of using PZTs 12, wherein I, V, N, and Ron represent power source current, power source voltage, number of simultaneously-driven ink jet nozzles nozzle resistance value (on-resistance of a switch in the driver IC), respectively.
Assuming that the current per nozzle is 20 mA and that there are 128 channels in the printer head 1, then the total current is 2.56 A. Further, assuming that the resistance value of each conductor strip in the power supply conductors 5 is 1.OMEGA., then the resistance value of the supply and return conductive strips of the power supply conductors 5 is 2.OMEGA.. Thus, the voltage drop in the power supply conductors 5 is 5.12 V. The voltage supplied actually to the terminals decreases as shown by the dotted line in FIG. 17. This voltage drop caused by the power supply conductors 5 is not negligible relative to the power source voltage (e.g., 24 V). As a result, the electric power that should be applied to the printer head 1 is consumed wastefully by the power supply conductors 5.
Further, since the power supply conductors 5 include inductance as well as resistance, the sharp rising and falling of the voltage applied to the driver IC are lessened as shown by dotted line in FIG. 18.
Still further, since the input-output characteristics of the driver IC 2 is nonlinear, a head drive signal is distorted further. More specifically, the driver IC 2 which is typically C-MOS analog switches has a non-linear switch resistance versus input voltage characteristics as shown in FIG. 19. The analog switch has a function to turn on/off the input and output in response to the control signal. Due to its nonlinear characteristics, the input and output resistance provided when the switch is turned on is changed relative to the input voltage. Therefore, as shown in FIG. 20, as compared with the output from the pulse generator 42, a large delay occurs in the leading edge and the trailing edge of the output from the driver IC 2. Thus, it frequently occurs that the printer head 1 cannot demonstrate its intended ink jet ability fully. That is, an ink-jet speed is lowered lowering a printing quality.
To reduce the loss occurring in the power supply conductors 5, the width of the conductor strip be increased in order to lower the impedance of the power supply conductors 5. Further, a capacitor 10 of large capacity and a capacitor of low impedance should be disposed near the driver IC 2. Those additional circuit elements will cause the control system to become large-sized and expensive.